Sounds to me like something in the START-CHARGE-RUN circuitry is wrong, I'm going to
guess the voltage regulator and/or alternator are hooked up incorrectly and the fusible links
have been disabled or bypassed. Look for burn wires in that area under the hood. If you've
got burn wires inside the cab, my best advice is to tow it home as diagnosis & repair is
likely to take significant time.

Where are you at (city/state/country)? If you can, click on User CP and fill out the part of
your profile pertaining to where you're located so we can see it.